1. Field of the Invention
This invention relates to the alignment of active optical components with optical waveguides. In particular, though not exclusively, the invention relates to the alignment of an active optical component with respect to a planar optical waveguide chip.
2. Description of the Related Art
In the field of optical communications there is increasingly a trend towards so-called integrated planar-waveguide devices, in which one or more active optical components are mounted on a waveguide chip. Each optical component needs to be aligned with respective optical waveguides. An example of such an integrated planar-waveguide device is a hybrid integrated optical gate matrix switch, which uses an array of semiconductor laser amplifiers optically aligned with an array of waveguides to open or close respective optical circuits.
Planar waveguide devices typically include a substrate, such as silicon, quartz or lithium niobate, one or more waveguide regions (equivalent to the core of an optical fibre), and, surrounding the waveguide regions, one or more cladding regions or layers. In addition, one or more buffer layers may lie between the substrate and the waveguide regions. To achieve waveguiding, it is useful for the waveguide regions to be made of a material or materials having a higher refractive index or indices than the refractive index (or indices) of the cladding region(s). While plastic materials can be used, currently silica and doped silica are preferred.
The waveguides are typically of square or rectangular cross section with typical side lengths of 5 to 8 micrometres. Typically the cladding and buffer layers each have a thickness approximately three times that of the waveguides.
In many planar waveguide devices, there are multiple waveguides and these need to be aligned with and coupled to multiple optical components. The importance of accurate alignment of optical waveguides with other optical components is well known. In aligning an optical waveguide with an active optical component, not only must the waveguide and active layer of the component be aligned angularly and in three co-ordinates for optimum coupling, but this alignment must also be fixed securely and must not be altered during fixation or subsequent processing.
A number of prior art techniques for mounting and aligning optical components with planar waveguide chips are disclosed in U.S. Pat. No. 6,164,836. In order to provide accurate alignment of waveguides with optical components, positioning reference surfaces are required on both the waveguide chip and the optical component suitable for establishing references along three axes. In all of the examples presented in U.S. Pat. No. 6,164,836, a surface of the substrate of the waveguide chip is employed as a xe2x80x98heightxe2x80x99 reference surface on which the optical component is mounted to align the level of the component with that of the waveguide regions in a direction perpendicular to the plane of the substrate. Where active optical components are to be used, the requirement for electrical contacts on the waveguide chip means that the height reference surface of the substrate is provided by a terrace formed by a protrusion of the substrate through an electrical contact layer. At positions where a height reference is required, mesas must be formed on the substrate prior to deposition of the silica cladding and waveguiding layers. Hence, the design of the chip must be set at an early stage of manufacture. After subsequent deposition of the cladding and waveguiding layers, careful selective removal is required to expose the mesas to provide the height references for the active components.
Each processing step involved in the manufacture of planar waveguide devices increases costs and the risk of defects, and clearly the later in the manufacturing process that defects become apparent, the greater the cost of wastage. It is therefore desirable to minimise the number of processing steps.
It is an object to produce an improved connector system for passive alignment of active optical components with optical waveguides.
According to a first aspect of the present invention, there is provided a connector assembly for connecting and aligning an active optical component with an optical waveguide, the assembly comprising:
(i) a waveguide chip having an optical waveguide embedded beneath a cladding layer and a cavity for accommodating the active optical component delineated by at least one wall extending from the surface of the cladding layer through the waveguide; and
(ii) a second chip for carrying the active optical component,
wherein the waveguide chip comprises a locating stop and the second chip has first and second reference regions formed thereon, the first reference region being adapted to locate the active optical component, and the second reference region being adapted to engage the surface of the cladding layer and the locating stop of the waveguide chip when the waveguide chip and second chip are connected together with the active optical component located within the cavity in order to provide alignment of the waveguide with the active optical component.
The inventor has found that the surface of the cladding layer can be used to provide an accurate xe2x80x98heightxe2x80x99 reference with respect to the waveguide region which can be used by the second chip to assure alignment with the active layer of the optical component. Since there is no need for the substrate of the waveguide device to provide a height reference, there is no need to form a mesa for this purpose. In this way, the number of process steps required to manufacture the device is significantly reduced, together with associated costs of manufacture. Yield wastage due to defects is reduced due to the reduced number of process steps. In particular, since the number of process steps prior to connection of the active components is reduced, the cost of wastage due to problems at the connection step is significantly reduced.
Furthermore, since it is not necessary to form mesas on the waveguide device, the ultimate location of active components on the waveguide device does not need to be decided until patterning of the core layer of the waveguide device. Hence, it may be possible to use the same waveguide device substrate as a building block for different components.
Suitably, the cladding layer of the waveguide chip and the second reference region of the second chip comprise planar surfaces. The first reference region of the second chip may comprise a locating stop formed on the planar surface.
Suitably, an active optical component is mounted and glued in place on the second chip. Alternatively, the component may be soldered in place. The waveguide chip and the second chip may be glued together.
According to a second aspect, the invention provides a method of connecting and aligning an active optical component with an optical waveguide embedded within a waveguide chip beneath a cladding layer, the method comprising the steps of:
(i) Forming a cavity in the waveguide chip extending from the surface of the cladding layer through the optical waveguide for accommodating the active optical component;
(ii) providing a locating stop on the waveguide chip;
(iii) forming first and second reference regions on a second chip, the first reference region being adapted to locate the active component, and the second reference region being adapted to engage the surface of the cladding layer and the locating stop;
(iv) mounting the active optical component on the second chip;
(v) connecting the second chip to the waveguide chip such that the second reference region engages the surface of the cladding layer and the locating stop such that the active optical component is located in the cavity and in alignment with the waveguide.
Preferably, the cavity is formed in the waveguide chip by deep etching or precision milling. After precision milling the cavity, no further process steps are required prior to connection of the active optical component.